Implement alternative orders for GLMModifier to write out the model to file

[trevorhardy]

Currently GLMModifier writes out the model to file by class (e.g. all houses grouped together, all lines grouped together). In some cases this is fine and in other cases, for human readers of the file, other orders for the objects on the page would be preferable. We need to evaluate and define these alternative methods and get them implemented in GLMModifier.

[trevorhardy]

I think we have a graph-based way of doing this now; is that correct?

[trevorhardy]

Not yet implemented and there are other alternatives (e.g. writing out configurations in separate files). Need to do some more thinking before we dig into this.